Abstract

An explanation of galvanic corrosion and the galvanic series. This guide explains the effects of galvanic corrosion in roofing applications.

Galvanic corrosion and roofing

Galvanic Corrosion On a Roof

When designing and constructing metal roofs it is important to consider the different qualities of the metals employed. This is particularly important when two different metals are used on the roof together. Metal roofing often incorporates more than one type of metal in its construction. A prime example is where a roof covering meets a roof flashing.

Galvanic Corrosion

The relationship between Galvanic couples.

Galvanic Table or Series

When two different metals come into contact with each other in the presence of water, an electrical current will flow between them. These two reacting metals are called galvanic couples. It is important for the roofing contractor to understand the relationship between Galvanic couples.

When the two metals meet, one metal will donate electrons known as the anode. This metal will suffer from corrosion at a faster rate. Conversely, the second metal will receive electrons (called the cathode) and will undergo a slower rate of corrosion. The degree to which this process takes place is dependant on the difference between the two metals electrochemical potentials. To help determine the electrochemical potential of two metals we can turn to the galvanic table or series. The galvanic series is a list or table that shows metals according to their electrochemical potential. One end of the list has the most active metals, called Anodic or least noble metals. The opposite end has passive metals (Cathodic) or Noble metals. The further apart the two metals are on the table the higher will be the potential for galvanic corrosion. Correspondingly, metals that are closer together will experience less corrosion.

Anodic, Active or least noble metals This End

Zinc

Aluminium

Cadium

Steel Iron

Aluminium Bronze

Brass

Tin

Copper

Lead Tin Solder

Admiralty Brass

Manganese Bronze, Silicone Bronze

Stainless Steel (passive type 410-416)

Nickel Silver

High Copper Content Nickel Copper Alloys

Lead

Nickel Aluminium Bronze

Nickel

Silver

Type 304 and 316 Stainless Steel

Titanium

Most Cathodic or noble metals this end

The Galvanic corrosion Process

Metals listed higher on the table will donate electrons (anode) if coupled with a metal lower on the list (cathode). Put more simply, a metal higher on the table will experience corrosion if coupled with a metal lower on the table. The metal lower on the table will experience less galvaniccorrosion.

Galvanic corrosion rates will be increased where environmental conditions promote the flow of electricity between metals. So salt laden air and areas of high pollutants will experience an increase in galvanic corrosion.

Applications of Galvanic Corrosion

A metal higher on the galvanic table will “sacrifice” itself and reduce corrosion of the lower metal. Iron made storm water holding tanks suffer from serious corrosion in a marine saltwater environment. The corrosion is reduced by fixing large blocks of Zinc to the iron structure. The Zinc corrodes badly but the iron is protected by the galvanic action.Cathodic protectionis used in roofing.Corrugated iron roofing is protected by the Zinc coating. Even if the coating is scratched revealing bare metal, the galvanic process will protect the Iron.

Water run off and galvanic corrosion

There areroofing situations where two metals do not actually come into contact with each other yet galvanic corrosion still occurs. This happens when tiny minute particles of metal are carried by rainwater over the surface of the second metal. So for instance, a copper roof that drains into galvanised iron guttering/spouting will quickly corrode the guttering. Similarly, lead flashings draining onto a corrugated iron roof will also corrode the roof surface. So in situations where water flows from one metal type over another, the first metal should be Anodic or higher on the table.

Preventing Galvanic Corrosion

In order for

galvanic corrosionto take place, electricity must flow between two different metals. By insulating the metals with a galvanic sheath the process is halted. Roofing felt or bitumen paper can be used to separate the metals. Galvanic paint with a high Zinc content can also be used on both surfaces.

In summary

Galvanic corrosion occurs when two different metals meet in the presence of water. A metal higher on the galvanic table will suffer more galvanic corrosion than a noble metal lower on the table. The further apart the two metals are on the galvanic table the worse will be the corrosion.